Process for descaling titanium



1957 I w. J. WALLACE- PROCESS FOR DESCALING TITANIUM Filed Sept; 6, 1955 INVENTOR. W. J. WALLACE A TTo'RNEr r/// w r// PROCESS FOR DESCALING TITANIUM Wilfred J. Wallace, Chula Vista,Calif., assignor to Rohr Aircraft Corporation, Chula Vista, Calif., a corporation of California Application September 6, 1955, Serial No. 532,538

- 6 Claims. (Cl. 148-614) This invention relates to a process for treating metal parts made of titanium which have to be heated to form theparts to a desired shape or for other purpose and during which treatment oxides are formed on the parts which are normally very difiicult to remove. It is believed that the oxide film usually formed when the uncoated metal is heated to from 400 to 1400 F. consists largely of the higher oxides of the metal such at TiOz and TiO3 and that such oxides are difiicult to later remove.

Furthermore oxidation in the presence of air proceeds rapidly at high temperatures so that a rather thick layer of oxides is formed thereby detracting from the strength of the part. This impairment of strength is especially marked when the parts are formed of thin sheet titanium having a thickness from .012 to .050 inch.

It is an object of this invention to provide a treatment for one or more titanium parts which require heat treatment which assures the formation of thin film of oxide or oxides of titanium on the surface of the metal, during the heating, which film can later be completely and easily removed.

Another object is to provide a treatment for a titanium part or parts which require heat treatment up to a temperature of from 900 to 1400 F. which forms a thin protecting film on the parts consisting of the lower oxides of titanium, this film'being formed while the temperature of the parts is increasing within the range of approximately 400 .to 500 F. and protecting the metal from further oxidation as the temperature of the parts continues to increase.

A further object is to provide a treatment which will quickly and completely remove the scale formed during the heating operation and leave the exposed surface of the finished part cleanand free from scale.

Further objects will become apparent as the description of the treatment proceeds. For a better understanding of the invention reference is made to the accompanying drawing, in which:

Fig. l is a sectional View of a tank containing a titanium sheet immersed in a treating liquid in the tank;

Fig. 2 is a diagrammatic sectional view of a heating furnace;

Fig. 3 is a diagrammatic view, partly in section of a forming press used in the process and;

Fig. 4 is a sectional view of a tank containing a treated sheet immersed in a descaling bath.

The sheet of titanium to be treated is first dipped in a bath 11 contained in a tank 12 made of material which is not affected by the chemicals in the bath. The bath is preferably kept at a temperature of from 75 to 80 F. by any suitable heating means as,'for example, a steam pipe13 passing through the tank.

The bath is made by heating water to the temperature named, adding sodium or potassium nitrate and sodium or potassium carbonateand stirring or agitating till the ture of both oxides.

e 2,804,407 Patented Aug. '27, 1957 alcohol is added and the bath slightly agitated. These materials are added in suitable amounts to provide an aqueous solution of the following proportions by weight:

7 Percent Sodium or potassium nitrate About 40 Sodium or potassium carbonate 2 to 3 Alcohol 0.8 to 4.0 Sodium aryl sulfonate About 0.01

Water Instead of the alcohols mentioned any of the alcohols completely soluble in water and having a vapor pressure of from 40 to millimeters of mercury at 20 C. may be used instead. The sulfonate acts as a wetting agent by reducing the surface tension of the solution and facilitating the complete wetting of the sheet.

After the sheet is wet, it is removed from the bath and. allowed to dry in a current of slowly moving air at a temperature from 70 to F. The alcohol evaporates and assists the wet coating on the sheet to dry rapidly. The sheet is now coated with a thin film of material which will cause titanium oxides of desired composition to form when the sheet is heated to form it to a desired shape. 7

Heating The dry coated sheet or sheets are now heated to the required forming temperature in a furnace 14 which is opened by a door 15 sliding in guides 16. The interior of the furnace is heated to the required forming temperature usually within the range of 400 to 1200 F. depending on the extent to which the sheet has to be bent to conform it to the desired shape by the forming die. This heating may be done by any suitable means such as electric heaters 17 embedded in the floor or wall of the furnace. As the heating proceeds and the sheet passes from 400 to 500 F., oxygen is liberated from the alkali nitrate, this oxygen uniting with the titanium to form a thin film composed essentially of the lower oxides of the metal, believed to be TizOs, TiaOt or a mix- The carbonate present lowers the fusion point of the mixture on the, metal and also has a reducing action which limits the formation of higher oxides such as the dioxide TiOa and trioxide Ti03 whose presence is undesirable and would make complete removal ,of the scale from the articles more difiicult. Asv the sulfonate present is heated above 300 F. it decomposes and has a reducing action which tends to prevent the formation of the higher oxides of the metal. If it is necessary to. increase the temperature beyond 500 F., the film of the lower oxides of titanium which have been formed on the surface of the sheet acts as a barrier to prevent the oxidation from penetrating further into the metal and also prevents formation of the deleterious higher oxides such as TiO and TiO3.

Forming When the sheet has reached the required forming temperature, the door 15 ofthe furnace is raised, the coated sheet removed from the furnace and the hot sheet immediately put on female forming die 18. This die is supported on a fixed anvil 19 and may be at room temperature. When a deep draw is required however, the die is preferably heated, as by electric heating coils 20. embedded in the die, to a temperature of from 400 to 800 F. The male die 21 is immediately lowered to press the sheet against die 18 and'form it to the desired shape. Die 21 is shown as mounted on a ram 22 which is. con nected to a piston rod 23 operated by a piston 24 slidable ton from pipe 27. The usual vertical guides (not shown) Balance forming, compressed air is admitted to pipe. 27 which.

raises the die 21 when the formed sheet is removed and allowed to cool to roomtemperature (about 70 F.).

The formed sheet is nowready for having-the adherentl scale removed from itshsurface.-

Descaling The removal of the scale is effected by immersing the Percent Nitric acid About 29 Ammonium bifluoride, NH4HF2 About 2. Water s Balance Instead of the ammonium bifluoride an equal weight of sodium fluoride may be used instead or about 2.5. percent.

of technical grade 60 percent hydrofluoric acid '(HF) may be mixed with the nitric acid solution. 2

Upon removal from bath 28, the sheet is rinsed in water to remove the descale solution and allowed to dry.

The surface of the finished sheet is of uniform color and.

free from scale which would detract from its appearance.

An alternative method for securing clean formed sheets of'good appearance is to first heat the sheet without any previous treatment in furnace 14 to the forming temperature (600 to 1400 F.). This heating produces an oxide film or coating of the higher oxides of titanium .TiOz and/r TiOz on the sheet. is placed on die 18 and formed in the manner abovedescribed. The sheet is then allowed to cool and anygreasev or other lubricant which was used on it to facilitate the forming is removed in a known manner. The oxide coated sheet is then dipped in bath 11 of the composition above described, removed from the bath and allowed to dry. The formed sheet is now placed in furnace 14 and The coated sheet while hot.

shape of a curved die which comprises the steps of dissolving a salt of the group consisting of sodium nitrate and potassium nitrate and a salt of the group consisting of sodium carbonate and potassium carbonate in water; heating the aqueous solution to a temperature within the range of 75 to 80 F.; dipping the entire sheet in the heated solution; drying the wet sheet to leave a thin oxygen-containing coating thereon; heating the coated 'sheet in the presence of air to a temperature within the range of 400 to 1200 F. to oxidize the surface thereof;

and pressing the hot. oxidized sheet against said die.

2. The method of treating a formed sheet of titanium which has previously been heated to a high temperature and is. coated with a surface film composed ofthe higher oxides of titanium, which comprises the. steps of dipping the entire sheet in an aqueous solution of a salt of the group consisting of sodium nitrate and potassium nitrate and a salt ofthe group consisting of sodium carbonate and potassium carbonate; dryingthe wet coated sheet; and heating the dry coated sheet to a temperature higher than 400 F. to reduce the higher oxides of titanium in thesurface film,

3. The method of forming a titanium sheet to the con- 0 tour of a curved die which comprises the steps of treating the sheet with a salt solution at a temperature about 75 F., said solution consisting essentially of a salt of the group consisting of sodium nitrate and potassium nitrate,

a salt of the group consisting of sodium carbonate and potassium carbonate, and water; drying the wet sheet to leave .a thin oxygen-containing coating thereon; heating the coated sheet to a temperature above 400' F. to soften the sheet for forming and oxidizing the surface thereof; pressing the heated sheet against the face of the die; re-

- moving. the. heated sheet from the die; and treating the formed sheet with an aqueous solution of nitric acid and ammonium bifluoride to dissolve the scale on the sheet. 4. The method of forming a thin sheet of titanium to theyshape of a curved die face which comprises the steps reheated to the forming temperature. This reheat causes l ,the chemicals on the oxide film to reduce the higher.

oxides therein to the lower oxides (TizOa and/or Ti304) and also relieves the sheet of any internal stresses. which may have been set up therein by the forming. The'sheet is now removed from the furnace, placed on die18 again i formed sheet is ready for sale or for assembly with other parts into a composite structure.

While the above description has referred to the treat.- I

ment of a single sheet, it will be understood that a number of sheets may be treated in baths 11 and 28 simultaneously and also that a number of sheets maybe heated in furnace 14 simultaneously. a

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiof wetting the entire sheet with an aqueous solution consisting essentially of sodium nitrate about 40 percent, sodium carbonate about 2 to 3 percent, balance water; drying the wetted sheet to provide a thin dry surface film thereon; heating the treated sheetto a temperature within the range of 400 to 1200 F. to provide a thin surface coating of the. lower oxides of titanium thereon; and pressing the hot coated sheet against said dieface.

5. T hemethod forming a thin sheet of titanium to the shape of a curved die face which comprises the steps of wetting the entire sheet with an aqueous solution consisting essentially of potassium nitrate about 40 percent,

potassium carbonate about 2 to 3 percent, balance water; drying the wetted sheet to provide a thin dry surface film thereon; heating the treated sheet to a temperature within the range of 400 to 1200 F. toprovide a thin surface coating of the lower oxides of titanium thereon; and pressing the hot coated sheet against said die, face.

6. The method of forming a sheet of titanium to the shape of a curved die which comprises the stepsof dissolving a salt of the group consisting of sodium nitrate and potassium nitrate and a salt of the group consisting of sodium carbonate and potassium carbonate in water to provide a bath; adding about 1 percent of alcohol whose vapor pressure is between 40 and 80 millimeters of merment of the invention is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims,

and all changes which come within the 'meaning; and range of equivalency of the claims are intended to be cury at 20 C. and about 0.01 percent of a soluble surface tension-reducing compound to the bath; dipping the entire sheet in the bath; drying the wet sheet to leave a thin oxygen-containing coating thereon; heating the coated sheet in the presence of air to a temperature within the 'range of 400 to 1200 F. to oxidize the surface of the sheet; and pressing the hot treated sheet against said die.

References Cited in the file ofthis patent 4 UNITED STATES PATENTS 2,289,443 Monge July 14, 1942 (Other references on following page) 5 6 UNITED STATES PATENTS FOREIGN PATENTS Bergin Feb. 2, 1943 734,859 Germany Apr. 29, 1943 Mabus Mar. 7, 1950 Mayfield May 22, 1951 REFERENCES caugherty Ju 3 1952 5 Handbook on Tltamum by Adenstedt, August 1954, Hamilton etal Mar. 29, 1955 Pages McPherson Nov. 22, 1955 

4. THE METHOD OF FORMING A THIN SHEET OF TITANIUM TO THE SHAPE OF A CURVED DIE FACE WHICH COMPRISES THE STEPS OF WETTING THE ENTIRE SHEET WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF SODIUM NITRATE ABOUT 40 PERCENT, SODIUM CARBONATE ABOUT 2 TO 3 PERCENT, BALANCE WATER, DRYING THE WETTED SHEET TO PROVIDE DA THIN DRY SURFACE FILM THEREON; HEATING THE TREATED SHEET TO A TEMPERATURE WITHIN THE RANGE OF 400* TO 1200*F. TO PROVIDE A THIN SURFACE COATING OF THE LOWER OXIDES OF TITANIUM THEREON; AND PRESSING THE HOT COATED SHEET AGAINST SAID DIE FACE. 